The present invention relates to a method of fabricating an aperture array used for splitting an electron beam into a multiplicity of child beams and deflecting the child beams independently of each other, a wet etching method and a wet etching apparatus for fabricating the aperture array, and an electron beam exposure apparatus of a blanking aperture array (BAA) type having the aperture array thus fabricated. In recent years, semiconductor technologies have developed to such an extent that the improvement in the integration degree and the functions of the semiconductor integrated circuits (IC) is expected to play a critical role in the progress of the technologies of the industries as a whole including the computer and the communication system control. The integration degree of ICs has quadrupled every two or three years. The storage capacity of the dynamic random-access memory (DRAM), for example, has increased from 1 M to 4 M to 16 M to 256 M to 1 G. This increased integration degree depends to a large measure on the progress of microfabrication technology for semiconductor fabrication.
Under the circumstances, the limit of the microfabrication technology is defined by the pattern exposure technology (lithography). Pattern exposure technology presently uses an optical exposure apparatus called a stepper. In the optical exposure apparatus, the minimum width of the pattern that can be formed is defined by the wavelength of the exposure light source due to the diffraction. At present, a light source for emitting the ultraviolet ray is used for the optical exposure apparatus. Nevertheless, it is difficult to use the light of a shorter wavelength than ultraviolet. For realizing more finely detailed microfabrication, therefore, a new exposure system, other than the optical exposure apparatus, has been under study.
In the electron beam exposure process, microfabrication on the order of 0.05 .mu.m or less is known to be realized with an alignment accuracy of not more than 0.02 .mu.m. The conventional electron beam exposure method, however, is lower than the stepper in throughput and has been considered impossible to use for mass production of LSIs. The invention in recent years of what is called the block exposure method (the partial pattern collective transfer method, in which repetitive patterns are exposed by collective transfer and the remaining patterns are exposed with variable rectangles) is expected to achieve a high throughput for devices such as memories involving many repetitive patterns. With logic devices or the like having a random pattern, however, the advantage of using the block exposure method is so low that an improved throughput is still difficult to achieve. Thus, the development is desired of an electron beam exposure method in which a high throughput exposure is possible for devices having a random pattern.
The conventional electron beam exposure method uses a single electron beam. Even the block exposure method requires the use of a single electron beam for a random pattern, and fails to achieve a sufficiently high throughput. Also, the application of the block exposure method to low-item-count multi-production devices requires a multiplicity of block patterns, and the increased number of block patterns limits the scope of application of the block exposure method. In view of this, an exposure method of a multi-beam type has been proposed in which the exposure is carried out by use of a plurality of independently controllable electron beams generated by a blanking aperture array (BAA). The present invention relates to a method of fabricating a BAA, a wet etching method and a wet etching apparatus for fabricating the BAA, and an electron beam exposure apparatus having the BAA thus fabricated. The exposure method of this type is called a blanking aperture array (BAA) method herein. The present invention, however, is not limited to a method of fabricating the BAA, a wet etching method and a wet etching apparatus for fabricating the BAA, and an electron beam exposure apparatus of BAA type, but is also applicable to any aperture array having an electrostatic deflector on the sides of each aperture of a substrate and a wet etching method and a wet etching apparatus for fabricating a substrate formed with through holes. In the description that follows, an electron beam exposure apparatus of a BAA type will be dealt with as an example.
In an electron beam exposure apparatus of a BAA type, the electron beam emitted from an electron gun is applied to a BAA device and converted into a multiplicity of fine beams each of which is controllable independently of each other. These fine beams are deflected by a main deflector and a subdeflector and radiated at a desired position on a specimen placed on a stage. At the same time, the fine beams are each focused into a small spot on the specimen by an electromagnetic lens. The fine beams are deflected to scan the specimen and each fine beam is turned on and off in synchronism with the deflection thereby to produce a desired exposure pattern. The electron beam exposure apparatus of BAA type, in addition to the advantage that an arbitrary pattern can be exposed freely, has another advantage in that the continuous change of the scan signal eliminates the time of setting up the beam required for the vector scan and therefore makes possible a high-efficiency exposure by a high-speed scan. Further, the same spot is exposed a plurality of times for a high exposure energy.
The BAA device is formed of a thin substrate such as a silicon wafer. The substrate has a multiplicity of apertures arranged two-dimensionally each having an electrostatic deflector. The electrostatic deflector is configured of a pair of parallel electrodes disposed on the two sides of each aperture on the substrate. The electron beam that has entered the BAA device is split into fine beams as it passes through the apertures. Each fine beam passes through the corresponding aperture and irradiates a specimen when no voltage is applied to the corresponding electrode pair. Upon application of a voltage between the electrode pair, on the other hand, an electric field is formed and the fine beam is deflected. The fine beam thus deflected is shut off by a restrictor and therefore is not radiated onto the specimen.
The block mask used in the block exposure method and the BAA device used in the BAA method are fabricated on a silicon (Si) wafer about 500 to 600.mu.m thick. The fabrication process includes the step of partially etching the Si wafer to a thickness of several tens of .mu.m. For this purpose, the wet etching method is used in which the Si wafer is dipped in an etching solution.
In the process for fabricating semiconductor devices, wet etching is widely used as well as dry etching, wet etching is so called because of its wetness. A recently-developed process for fabricating a highly integrated semiconductor device mainly uses dry etching. For semiconductor integrated circuits (IC) of a comparatively large size, however, wet etching is used for forming a pattern or boring holes in a dielectric layer. Wet etching is also used for MEMS (Micro-Electro-Mechanical Systems), i.e. the process of fabricating what is called a micromachine. In the conventional wet etching method, the Si wafer to be etched is dipped in an etching solution. When carrying out the wet etching, a silicon oxide film (hereinafter called simply the oxide film) or a silicon nitride film (hereinafter called simply the nitride film) is used as a mask (protective film) for protecting the non-etched portion. These films have a lower etching rate than Si, and as far as it is formed to a thickness considering the selective etching ratio with respect to Si, the non-etched portion is not exposed to the etching solution.
In the conventional method of fabricating a BAA device, a wiring pattern is formed on a substrate, a dielectric layer is formed on the assembly in such a manner as to cover the wiring pattern, a plurality of recesses corresponding to a plurality of apertures are formed in the substrate, a plurality of contact holes adjoining the recesses, respectively, are formed in the dielectric layer for exposing the wiring pattern, a conductive film pattern is deposited in such a manner as to cover a plurality of the contact holes on the dielectric layer, an electrode of an electrostatic deflector electrically connected to the wiring pattern is formed in each of the contact holes by plating with the conductive film pattern as an electrode, the reverse side of the assembly is removed to a predetermined position by wet etching, and then the conductive film is removed. In this case, the surface conductive film functions as a protective film so that only the reverse side is wet etched. After the wet etching process, the surface conductive film is required to be removed. A conductive film of tantalum molybdenum, for example, is etched off using CF.sub.4 as an etching gas.
As the result of actual study conducted on the BAA device fabricated by the method described above, it has been found that the lower surface of the substrate is fouled and damaged by impurities. These defects are considered to derive from the fluoride formed by the reaction between the CF.sub.4 etching gas and SiO.sub.2 used as a specimen holder of the etching apparatus in the process of etching off the conductive film. In the case where a BAA device having these defects is arranged in an electron beam exposure apparatus, a charge-up is liable to occur in the defective portions, thereby leading to the problem that such defects as a distortion and a deformation develop in the exposed pattern. Thus, a method of fabricating a BAA device free of such defects is desired. It is also desired to improve the fabrication efficiency by improving the low efficiency of etching off the conductive layer using the CF, etching gas.